1. Field of the Invention
The present invention relates to zirconia ceramics and is more particularly concerned with zirconia ceramic compositions superior in bending strength, acid resistance and thermal stability.
2. Discussion of the Background
Zirconia ceramics containing 1.5-5.0 mol% yttria as a stabilizer are each known in the art as a partially stabilized zirconia ceramic (PSZ) which has been developed as a high strength zirconia ceramic useful for mechanical structure materials. The partially stabilized zirconia ceramic is, however, still insufficient in bending strength. In Japanese Patent Early Publication No. 60-226457, there has been proposed a method producing a high strength zirconia ceramic the bending strength of which is higher than that of the partialy stabilized zirconia ceramic. In this method, raw material fine powder with a specific primary mean particle diameter was mixed at a predetermined ratio and made by Hot Isostatic Pressing or sintered by uniaxial pressing to produce a ceramic composition consisting essentially of ZrO.sub.2 containing 1.5-5.0 mol% Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3 -MgO as a stabilizer.
During change of the ambient temperature from a high temperature to room temperature, the zirconia ceramic in the cubic structure form transforms into the tetragonal structure then to the monoclinic structure form with consequent volume changes. When the crystal structure changes on cooling from tetragonal to monoclinic, the zirconia ceramic is degradated due to a volume expansion thereof. For the purpose of preventing such degradation, there has been proposed a method of restraining the transformation of crystal stucture in ZrO.sub.2 by solid solution of a stabilizer such as CaO, MgO, Y.sub.2 O.sub.3 and the like. At present, Y.sub.2 O.sub.3 is used as the stabilizer to produce a partially stabilized zirconia ceramic of high strength and fracture toughness in the tetragonal structure form at room temperature. Such partially stabilized zirconia ceramics are, however, unstable in crystal phase and transform into the monoclinic structure when heated at a relatively low temperature within a range of 200.degree.-400.degree. C., resulting in deterioration in strength, fracture toughness and thermal stability thereof.
To enhance the strength and thermal stability, a zirconia ceramic composition consisting essentially of ZrO.sub.2 containing Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3 has been proposed in Japanese Patent Early Publication No. 58-32066, and a zirconia ceramic composition consisting essentially of ZrO.sub.2 containing Y.sub.2 O.sub.3, CeO and Al.sub.2 O.sub.3 has been further proposed in Japanese Patent Early Publication No. 61-77665. In the former publication, a mixture of Al.sub.2 O.sub.3 and MgO was suggested, but the mixture ratio of the oxides was not confirmed. In addition, it has been found that the zirconia ceramic composition proposed in the former publication is not useful to enhance the thermal stability, whereas the zirconia ceramic composition proposed in the latter publication is not useful to enhance the strength.